Re: [PATCH] Convert struct pid count to refcount_t

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Sat, Mar 30, 2019 at 11:16:01AM -0400, Alan Stern wrote:
> On Fri, 29 Mar 2019, Joel Fernandes wrote:
> > On Thu, Mar 28, 2019 at 10:37:07AM -0700, Paul E. McKenney wrote:
> > > On Thu, Mar 28, 2019 at 05:26:42PM +0100, Oleg Nesterov wrote:
> > > > On 03/28, Jann Horn wrote:
> > > > >
> > > > > Since we're just talking about RCU stuff now, adding Paul McKenney to
> > > > > the thread.
> > > > 
> > > > Since you added Paul let me add more confusion to this thread ;)
> > > 
> > > Woo-hoo!!!  More confusion!  Bring it on!!!  ;-)
> > 
> > Nice to take part in the confusion fun too!!! ;-)
> > 
> > > > There were some concerns about the lack of barriers in put_pid(), but I can't
> > > > find that old discussion and I forgot the result of that discussion...
> > > > 
> > > > Paul, could you confirm that this code
> > > > 
> > > > 	CPU_0		CPU_1
> > > > 
> > > > 	X = 1;		if (READ_ONCE(Y))
> > > > 	mb();			X = 2;
> > > > 	Y = 1;		BUG_ON(X != 2);
> > > > 
> > > > 
> > > > is correct? I think it is, control dependency pairs with mb(), right?
> > > 
> > > The BUG_ON() is supposed to happen at the end of time, correct?
> > > As written, there is (in the strict sense) a data race between the load
> > > of X in the BUG_ON() and CPU_0's store to X.  In a less strict sense,
> > > you could of course argue that this data race is harmless, especially
> > > if X is a single byte.  But the more I talk to compiler writers, the
> > > less comfortable I become with data races in general.  :-/
> > > 
> > > So I would also feel better if the "Y = 1" was WRITE_ONCE().
> > > 
> > > On the other hand, this is a great opportunity to try out Alan Stern's
> > > prototype plain-accesses patch to the Linux Kernel Memory Model (LKMM)!
> > > 
> > > https://lkml.kernel.org/r/Pine.LNX.4.44L0.1903191459270.1593-200000@iolanthe.rowland.org
> > > 
> > > Also adding Alan on CC.
> > > 
> > > Here is what I believe is the litmus test that your are interested in:
> > > 
> > > ------------------------------------------------------------------------
> > > C OlegNesterov-put_pid
> > > 
> > > {}
> > > 
> > > P0(int *x, int *y)
> > > {
> > > 	*x = 1;
> > > 	smp_mb();
> > > 	*y = 1;
> > > }
> > > 
> > > P1(int *x, int *y)
> > > {
> > > 	int r1;
> > > 
> > > 	r1 = READ_ONCE(*y);
> > > 	if (r1)
> > > 		*x = 2;
> > > }
> > > 
> > > exists (1:r1=1 /\ ~x=2)
> > > ------------------------------------------------------------------------
> > > 
> > > Running this through herd with Alan's patch detects the data race
> > > and says that the undesired outcome is allowed:
> > > 
> > > 	$ herd7  -conf linux-kernel.cfg /tmp/OlegNesterov-put_pid.litmus 
> > > 	Test OlegNesterov-put_pid Allowed
> > > 	States 3
> > > 	1:r1=0; x=1;
> > > 	1:r1=1; x=1;
> > > 	1:r1=1; x=2;
> > > 	Ok
> > > 	Witnesses
> > > 	Positive: 1 Negative: 2
> > > 	Flag data-race
> > > 	Condition exists (1:r1=1 /\ not (x=2))
> > > 	Observation OlegNesterov-put_pid Sometimes 1 2
> > > 	Time OlegNesterov-put_pid 0.00
> > > 	Hash=a3e0043ad753effa860fea37eeba0a76
> > > 
> > > Using WRITE_ONCE() for P0()'s store to y still allows this outcome,
> > > although it does remove the "Flag data-race".
> > > 
> > > Using WRITE_ONCE() for both P0()'s store to y and P1()'s store to x
> > > gets rid of both the "Flag data-race" and the undesired outcome:
> > > 
> > > 	$ herd7  -conf linux-kernel.cfg /tmp/OlegNesterov-put_pid-WO-WO.litmus 
> > > 	Test OlegNesterov-put_pid-WO-WO Allowed
> > > 	States 2
> > > 	1:r1=0; x=1;
> > > 	1:r1=1; x=2;
> > > 	No
> > > 	Witnesses
> > > 	Positive: 0 Negative: 2
> > > 	Condition exists (1:r1=1 /\ not (x=2))
> > > 	Observation OlegNesterov-put_pid-WO-WO Never 0 2
> > > 	Time OlegNesterov-put_pid-WO-WO 0.01
> > > 	Hash=6e1643e3c5e4739b590bde0a8e8a918e
> > > 
> > > Here is the corresponding litmus test, in case I messed something up:
> > > 
> > > ------------------------------------------------------------------------
> > > C OlegNesterov-put_pid-WO-WO
> > > 
> > > {}
> > > 
> > > P0(int *x, int *y)
> > > {
> > > 	*x = 1;
> > > 	smp_mb();
> > > 	WRITE_ONCE(*y, 1);
> > > }
> > > 
> > > P1(int *x, int *y)
> > > {
> > > 	int r1;
> > > 
> > > 	r1 = READ_ONCE(*y);
> > > 	if (r1)
> > > 		WRITE_ONCE(*x, 2);
> > > }
> > > 
> > > exists (1:r1=1 /\ ~x=2)
> > 
> > I ran the above examples too. Its a bit confusing to me why the WRITE_ONCE in
> > P0() is required,
> 
> If the "WRITE_ONCE(*y, 1)" in P0 were written instead as "*y = 1", it
> would race with P1's "READ_ONCE(*y)".
> 
> >  and why would the READ_ONCE / WRITE_ONCE in P1() not be
> > sufficient to prevent the exists condition. Shouldn't the compiler know that,
> > in P0(), it should not reorder the store to y=1 before the x=1 because there
> > is an explicit barrier between the 2 stores? Looks me to me like a broken
> > compiler :-|. 
> > 
> > So I would have expected the following litmus to result in Never, but it
> > doesn't with Alan's patch:
> > 
> > P0(int *x, int *y)
> > {
> > 	*x = 1;
> > 	smp_mb();
> > 	*y = 1;
> > }
> > 
> > P1(int *x, int *y)
> > {
> > 	int r1;
> > 
> > 	r1 = READ_ONCE(*y);
> > 	if (r1)
> > 		WRITE_ONCE(*x, 2);
> > }
> > 
> > exists (1:r1=1 /\ ~x=2)
> 
> You have to realize that in the presence of a data race, all bets are
> off.  The memory model will still output a prediction, but there is no
> guarantee that the prediction will be correct.
> 
> In this case P0's write to y races with P1's READ_ONCE.  Therefore the 
> memory model may very will give an incorrect result.
> 
> > > ------------------------------------------------------------------------
> > > 
> > > > If not, then put_pid() needs atomic_read_acquire() as it was proposed in that
> > > > discussion.
> > > 
> > > Good point, let's try with smp_load_acquire() in P1():
> > > 
> > > 	$ herd7  -conf linux-kernel.cfg /tmp/OlegNesterov-put_pid-WO-sla.litmus 
> > > 	Test OlegNesterov-put_pid-WO-sla Allowed
> > > 	States 2
> > > 	1:r1=0; x=1;
> > > 	1:r1=1; x=2;
> > > 	No
> > > 	Witnesses
> > > 	Positive: 0 Negative: 2
> > > 	Condition exists (1:r1=1 /\ not (x=2))
> > > 	Observation OlegNesterov-put_pid-WO-sla Never 0 2
> > > 	Time OlegNesterov-put_pid-WO-sla 0.01
> > > 	Hash=4fb0276eabf924793dec1970199db3a6
> > > 
> > > This also works.  Here is the litmus test:
> > > 
> > > ------------------------------------------------------------------------
> > > C OlegNesterov-put_pid-WO-sla
> > > 
> > > {}
> > > 
> > > P0(int *x, int *y)
> > > {
> > > 	*x = 1;
> > > 	smp_mb();
> > > 	WRITE_ONCE(*y, 1);
> > > }
> > > 
> > > P1(int *x, int *y)
> > > {
> > > 	int r1;
> > > 
> > > 	r1 = smp_load_acquire(y);
> > > 	if (r1)
> > > 		*x = 2;
> > > }
> > > 
> > > exists (1:r1=1 /\ ~x=2)
> > > ------------------------------------------------------------------------
> > > 
> > > Demoting P0()'s WRITE_ONCE() to a plain write while leaving P1()'s
> > > smp_load_acquire() gets us a data race and allows the undesired
> > > outcome:
> > 
> > Yeah, I think this is also what I was confused about above, is why is that
> > WRITE_ONCE required in P0() because there's already an smp_mb there. Surely
> > I'm missing something. ;-)
> 
> A plain write to *y in P0 races with the smp_load_acquire in P1.  
> That's all -- it's not very deep or subtle.  Remember, the definition
> of a race is two concurrent accesses to the same variable from
> different CPUs, where at least one of the accesses is plain and at
> least one of them is a write.
> 
> I've heard that people on the C++ Standards committee have proposed
> that plain writes should not race with marked reads.  That is, when
> such concurrent accesses occur the outcome should be an undefined
> result for the marked read rather than undefined behavior.  If this
> change gets adopted and we put it into the memory model, then your
> expectation would be correct.  But as things stand, it isn't.

At least in the case where the marking is a volatile, yes.  But there
might be some distance between a proposal and an actual change to the
standard.  ;-)

There is also a proposal for a memcpy-like thing that acts as plain
loads and stores, but is defined not to data-race with marked accesses.
However, tearing and so on is possible, so if you do race with marked
accesses, you might be surprising results.

							Thanx, Paul